AgentAPI: State persistence #1256

Repo: coder/agentapi

Scope: Add state file support to AgentAPI binary so conversation history survives restarts.

Changes:

• Add --state-file <path> flag to agentapi server command
• Add --load-state flag (default true when state-file set)
• Add --save-state flag (default true when state-file set)
• Add --pid-file <path> flag so shutdown scripts can signal the correct process
• Implement Conversation.SaveState(path string) error
  • Use atomic write: write to temp file, then rename to target path
• Implement LoadState(path string) (*Conversation, error)
• Save state on SIGTERM, SIGINT, SIGHUP (skip if not dirty)
• Add SIGUSR1 handler to save without exit (for shutdown script coordination, can be called periodically)
• Track dirty flag: set on conversation change, clear on save
• State file format (JSON for schema compatibility):

  {
    "version": 1,
    "messages": [...],
    "initial_prompt": "...",
    "initial_prompt_sent": true
  }

Files (in agentapi repo):

• lib/screentracker/conversation.go
• cmd/server.go

Acceptance criteria:

• State saved to file on graceful shutdown
• State loaded from file on startup when flag set
• SIGUSR1 saves state without exiting process
• SIGTERM skips save if not dirty (no changes since last save)
• State file written atomically (temp file + rename)
• /messages endpoint returns restored history after restart
• Version field allows future format migrations

Dependencies: None

References:

• PRD: Start/Pause/Resume the Task Workspace
• RFC: Tasks: Start/Pause/Resume Lifecycle: AgentAPI State Persistence